TOKYO — Japanese officials took a series of early steps Friday to bring the Fukushima Daiichi nuclear power plant under control, but a week into the crisis, it was becoming apparent that they were confronting a problem that would not be resolved quickly.

Special military firetrucks were used to spray water at a damaged reactor building for a second day Friday in an effort to cool spent fuel in a pool whose water level was believed to have run dangerously low. One U.S. military firetruck was used in the nearly 40-minute operation along with six Japanese vehicles normally used to put out fires at plane crashes, officials said. All the vehicles were reportedly driven by Japanese.

A top U.S. nuclear official warned Thursday that the emergency could continue for weeks, while President Obama tried to reassure the American public about the safety of nuclear power plants in the United States.

The moves reflected widening worries in Japan and the United States about the failure so far to contain radiation leaks from nuclear power plants damaged in last Friday’s 9.0-magnitude earthquake and the devastating tsunami that followed.

Tokyo Electric Power Co., which owns the Fukushima Daiichi plant, said a risky mission using helicopters and water cannons Thursday to dump tons of water on the most troubled reactor had succeeded in reducing radiation levels. But Graham Andrew of the International Atomic Energy Agency cautioned at a news conference: “It is still possible that it could get worse.”

Japanese officials said they would continue trying to deliver water to storage pools. Without water, spent fuel rods stored in the pools would start to decay and release radioactive matter into the air.

There were also hopes that Japan’s success in reconnecting electric power to another reactor would allow engineers to restart pumps that play an essential role in delivering coolant.

Over the next few weeks, radiation will continue to spew from the plant at levels high enough to make it difficult for people to work there. What’s more, the facility itself has been ravaged by earthquakes, flooding and explosions that have torn much of the infrastructure — power lines, pumps and pipes — to shreds and scattered debris, making access even for robots challenging.

Toll continues to rise

As Japan continued its grim recovery effort, the official death toll from the earthquake and tsunami reached 5,692 people by Friday morning, with more than 9,500 others listed as officially missing. Nearly half a million are being housed in temporary shelters, and widespread power outages have left broad swaths of the country without adequate heat.

In Washington, President Obama made an unannounced visit Thursday to the Japanese Embassy and signed a condolence book. Later, speaking from the Rose Garden, he said the U.S. was “working aggressively to support our Japanese ally.”

The Yomiuri Shimbun, a leading Japanese newspaper, reported Friday that Japan’s government had turned down an early U.S. offer of help in cooling fuel rods at the damaged nuclear reactors. The paper reported that the government and Tokyo Electric believed that they would be capable of restoring the cooling system.

Government spokesman Yukio Edano denied that Japanese officials had rejected the offer.

As the crisis worsened, the United States took measures to protect Americans in Japan, sending buses to pick up several hundred citizens who had been stranded north of Sendai, in the heart of the quake zone.

The Pentagon said Thursday that it had sent a nine-member team of radiological specialists to Japan from the Colorado-based U.S. Northern Command to advise the Japanese military on responding to nuclear hazards.

The U.S. government has instituted stricter guidelines for its citizens in Japan, urging people to stay at least 50 miles away from the plant — four times the distance suggested by Japanese officials. On Thursday, several other nations joined the United States in adopting the 50-mile recommendation, including Canada, Britain and South Korea.

The diverging guidance fueled anxiety among some Japanese that they were not receiving reliable information.

Speaking at the White House, a top energy official acknowledged that the situation is confusing. “The facts on the ground are genuinely complex,” Deputy Energy Secretary Dan Poneman said.

Gregory Jaczko, chairman of the Nuclear Regulatory Commission, said the crisis at the Japanese facility “will likely take some time to work through, possibly weeks.”

Desperate missions

On Thursday, the Japanese government raced to treat the damaged Fukushima Daiichi plant by land and air, trying to cool two reactor units in particular that have raised alarm about the prospect of a larger, imminent catastrophe.

In an emergency mission Thursday, two Japanese military helicopters dropped more than 30 tons of water on the plant. Then, soldiers used 11 high-pressure fire trucks to douse a damaged reactor building from a distance. The soldiers acted after Japanese police failed in their efforts to spray the building with water cannons normally used for riot control.

Members of the Self-Defense Force, as the military is known, moved their trucks into position and began to spray water Thursday evening, taking aim at the same unit 3 reactor building that was targeted by helicopters earlier in the day. They sprayed water for more than half an hour before leaving the plant.

Edano said that officials believe that water did make it into unit 3, and a spokesman for Tokyo Electric said radiation levels showed a very small decrease after the helicopter missions. Noting the minuscule drop, the World Nuclear Association said the water drops “did not appear accurate enough to be effective,” adding that “the effect at present seems marginal at best.” The London-based organization, which promotes nuclear energy, said one attempt was made to douse the unit 4 reactor building but that the pilots withdrew “after encountering high levels of radiation.”

Japan has made unit 3 a priority because government officials say the storage pool contains less water than that of unit 4. American officials, however, have said they believe unit 4’s pool could be empty of water, and the International Atomic Energy Agency later said unit 4 remains a major concern.

An official with Japan’s nuclear safety agency also moved closer to the U.S. position on unit 4. “Considering the amount of radiation released in the area, the fuel rods are more likely to be exposed than to be covered,” the official, Yuichi Sato, said, according to the Associated Press.

Fear grows in region

As Japan struggled to avert a nuclear disaster, ripple effects from the crisis spread to other countries in the region.

In China, panic-buying swept from the country’s eastern coast all the way to Beijing, with residents rushing to stores to stock up on salt. People apparently believe the iodine in salt will protect them from radiation; others feared that sea salt would become scarce if the East China Sea becomes contaminated because of the nuclear crisis.

In Beijing and elsewhere, several supermarkets also ran out of imported milk powder, soybean sauce and instant noodles, as people stocked up on provisions even as the government issued repeated assurances that there was no radiation threat to China.

“I don’t know when I can replenish our stock,” said Chen Zhonghai, manager of the Jinli Super Market in Wenzhou City. “The residents worry that the salt produced in the future will be contaminated and can’t be eaten. It’s totally unnecessary.”

Stein reported from Washington. Correspondent Chico Harlan and special correspondent Akiko Yamamoto contributed to this report.